A conductive material exhibiting a resistance change according to a temperature change and a device using the same have been well known. A conventional PTC resistor has been known as a PTC thermistor using a doped BaTiO3 ceramic material. A thermistor made of the ceramic material exhibits a sharp PTC resistance effect at a higher temperature than its Curie temperature. Although the PTC device made of the ceramic material has long been used, it has a problem that it is restricted in applications and causes a high process expense because it has a relatively high resistance value at room temperature.
In an effort to solve the above problem, a conductive polymer composition that can be more easily fabricated compared to the conventional ceramic process, as well as which has a small resistance value at room temperature, has been developed. As examples, U.S. Pat. Nos. 4,237,441, 4,545,926 and 5,880,668 are given.
The conductive polymer compositions disclosed in the above documents exhibit. “PTC property” in which it has an electrical conductivity by uniformly dispersing carbon black or metal powder as a conductive filler into a polymer matrix, whereby its resistance is increased in proportion to a temperature rise, and its resistance is rapidly increased when the temperature goes up to higher than a certain point called a switching temperature.
The polymers used for the conventional PTC composition are mostly olefin-based polymers, for example, polyethylene (PE), polypropylene (PP), ethylene/propylene co-polymers and ethylene-based co-polymers such as ethylene(meta)acrylic acid co-polymers, ethylene ethyl acrylate co-polymers, ethylene butyl acrylate co-polymers and ethylene vinyl acetate co-polymers. Besides, polyvinyl-based co-polymers such as polyvinylchloride, polyvinylidenechloride, polyvinylfluoride, polyvinylidenfluoride, thermoplastic polymers such as polyamide, polystyrene, polyacrylonitrile, silicone resins, polyester, a modified cellulose or polysulfone may be used.
The PTC composition is typically used as a circuit protection device for limiting a current flow when a short-circuiting has taken place in the circuit comprising a heater, a positive character thermistor, a thermo-responsive sensor, a battery or the like, and for recovering the circuit to a normal state when the cause of the short-circuiting is removed. In addition, as an example of using the PTC composition, a PTC device, in which more than two electrodes are electrically connected to the PTC composition, can be given. The electrodes are connected to a power supply so that the current can flow through the PTC device. The PTC device is used as a protecting device for a circuit from current overload, overheating and the like, by functioning as a self-temperature controller as described above.
The device generally allows current to flow through a circuit since the resistance is low enough at a temperature below the switching temperature (Ts). However, at a temperature above the switching temperature, it does not allow any further current to flow, by rapidly increasing the resistance. In other words, when the circuit is heated up to a critical temperature, the PTC device functions as a circuit protecting device for decreasing a current overload caused by a short-circuiting to a lower and stable value. When the cause of the fault state is removed, the PTC device is cooled down below the critical temperature and returned to the low resistance state of its normal operation. Such effect is called a “reset”. The composition of which the PTC device is constructed is necessary to have such a current limiting performance and reset property allowing a repeated use at high voltage.
A polymer PTC electric circuit protecting device is generally formed by inserting a PTC component, which is fabricated by dispersing electrically conductive fine particles such as metal powder or carbon black into polymers, between a pair of electrodes. The electrodes are connected to a power supply so that the current can flow through the PTC device. In order to minimize a contact resistance, the electrodes are generally attached to the PTC composition by a thermo-fusion. However, in such methods, adhesion between components in the composition has been a problem. In order to overcome the problem, in the past, the surface of the electrodes was chemically or physically treated to be rough, or specially fabricated electrodes have been used (Japanese Laid Open Publication No. 5-109502 and U.S. Pat. No. 3,351,882, etc.). However, those methods have disadvantages in that the problem of contact resistance is not satisfactorily solved, and it is difficult to expect the repetition stability returning to the same resistance value as that of the initial stage even after several times of short-circuiting have taken place.
In addition, when a high working current is required even though its size is limited such as in a lithium ion battery, the PTC device to be inserted into the circuit is also limited in size. In general, in case of a PTC device, the maximum current value (that is, a hold current, IHmax), which is maintained at a normal working state without switching, differs according to the power consumption. The power consumption is related to an initial resistance of the device. The lower the initial resistance is, relatively the less the power consumption is, and accordingly, the PTC device can have a high maximum hold current. Thus, in the PTC device, as it has a high maximum hold current, in order to lower the resistance value of the device, the distance between a pair of electrodes is made short or the surface area of the electrodes has to be enlarged. If the space between the two electrodes becomes narrow, the resistance value of the device is also lowered down as much. However, if the space between the electrodes is too narrow, a PTC component constructed therebetween may easily be cracked by even a weak external impact, and it is not easy to manufacture, too. Therefore, in general, the area of the electrodes is enlarged while maintaining a certain thickness. In this respect, if the resistance value of the PTC component inserted between the electrodes is not low enough, the size of the formed device should be inevitably enlarged to larger than the limited circuit size to have a high hold current. In addition, if the contact resistance is high due to an insufficient adhesion, power consumption may be concentrated in the interface of the electrodes and the PTC component, and accordingly it is impossible to obtain the high maximum hold current.
In other words, the resistance value of the PTC component itself and the contact resistance between the electrodes and the PTC component should be low enough so as to retain a high hold current while allowing the PTC device to be inserted into a limited size of circuit to have a sufficiently small size. Also, in the case of the conventional PTC device using the conventional conductive polymer material, there has been a problem of reduced voltage characteristic when the resistance makes low in order to minimize the voltage drop. In order to solve such problems, a method of connecting two or more devices in parallel has been suggested. However, this method causes another problem in which a resistance increase at high temperature is also reduced when the resistance of the conductive polymer composition at room temperature is set to be low, and accordingly, the PTC intensity is reduced.
Therefore, it is still necessary to provide a PTC device having a sufficient PTC properties in which the resistance can be rapidly increased at high temperature while the resistance can be maintained low enough at room temperature.